1. Field of the Invention
The present invention relates to an optical communication device substrate having a negative thermal expansion coefficient and an optical communication device obtained by fixing an optical component having a positive thermal expansion coefficient onto the substrate.
2. Related Art
A network using an optical fiber has been rapidly improved in company with progress in optical communication technology. In the network, there has been used a wavelength multiplexing technique transmitting light with plural wavelengths collectively, so that a wavelength filter, a coupler, a waveguide and so on become important optical communication devices.
Among such optical communication devices, some have a trouble in outdoor use due to a change in characteristic according to a temperature; therefore a necessity has arisen for a technique to sustain a characteristic of such an optical communication device at a constant level regardless of a change in temperature, so-called athermal technique.
A fiber Bragg grating (hereinafter referred to as FBG) is exemplified as a representative of optical communication devices requiring athermalization. An FBG is an optical communication device having a portion with a profile of a changed refractive index in the form of a grating, so-called grating region, in a core of an optical fiber, and features reflection of light with a specific wavelength according to a relationship given by the following formula (1). For this reason, this has drawn attention as an important optical communication device in a wavelength division multiplex transmission optical communication system in which optical signals with different wavelengths are multiplex-transmitted through a single optical fiber.
xcex=2nxcex9xe2x80x83xe2x80x83(Formula 1) 
wherein xcex is a reflection wavelength, n is an effective refractive index in a core, and xcex9 is a spacing in a region with a changed refractive index in the form of a grating.
Such an FBG has a problem, however, that a center reflective wavelength fluctuates as temperature varies. A temperature dependency of a center reflective wavelength is given by the following formula (2), which is obtained by differentiating the formula (1) with respect to a temperature T.
∂xcex/∂T=2{(∂n/∂T)xcex9+n(∂xcex9/∂T)}=2xcex9{(∂n/∂T)+n(∂xcex9/∂T)/xcex9}xe2x80x83xe2x80x83(Formula 2) 
The second term of the right side of the formula (2), (∂xcex9/∂T)/xcex9, corresponds to a thermal expansion coefficient of an optical fiber, and the value thereof is almost 0.6xc3x9710xe2x88x926/xc2x0 C. On the other hand, the first term of the right side is a temperature dependency of a refractive index in a core portion of an optical fiber, the value thereof is almost 7.5xc3x9710xe2x88x926/xc2x0 C. That is, while the temperature dependency of a center reflective wavelength is dependent on both of a change in refractive index in a core portion and a change in spacing of the grating due to thermal expansion, most of a change in center reflective wavelength is found to be caused by a change in refractive index according to temperature.
As means for preventing a change in center reflective wavelength, a method has been known in which a tension adapted to a change in temperature is applied to an FBG to vary a spacing of a grating region, thereby canceling a component caused by a change in refractive index.
As a specific example, a device controlled with respect to a tension therein is disclosed in the Japanese Patent Laid Open No. 2000-503967, which device is fabricated this way: an FBG applied with a prescribed tension is fixed with an adhesive onto a glass-ceramic substrate having a negative thermal expansion coefficient, which is obtained by crystallizing a mother glass body molded into a plate in advance.
In the above device, the substrate shrinks with a rise in temperature, which reduces an applied tension in the grating region of an optical fiber. On the other hand, with a fall in temperature, the substrate stretches to increase an applied tension in the grating region of an optical fiber. In such a way, a tension applied to an FBG is caused to change according to a change in temperature to thereby enable a spacing of the grating in the grating region to be adjusted, with the result that a temperature dependency of a center reflective wavelength can be cancelled. It is also disclosed that while, in an optical communication device with such a substrate, glass, polymer or metal can be used for adhesion and fixing of FBG, polymer, especially, an epoxy resin adhesive, is suitable for fabrication of the device with a high efficiency.
Furthermore, in Japanese Patent Laid Open No. 2000-503967, the reason why this glass-ceramic substrate has a negative thermal expansion coefficient is described below.
Not only does the glass-ceramic substrate have a microcrack, but also includes a crystalline phase (xcex2-eucryptite solid solution) having a large negative thermal expansion coefficient along the c axis direction and a positive thermal expansion coefficient along the a axis direction. Additionally, the crystalline phase shrinks at the time of cooling along the a axis direction of a crystalline phase, but the shrinkage of the glass-ceramic substrate at the time of cooling is suppressed since clearances in the microcracks grow. On the other hand, the crystalline phase expands at the time of cooling along the c axis direction of the crystalline phase with no respect to microcracks. As a result, the glass-ceramic substrate has a negative thermal expansion coefficient because of a small contribution of a positive thermal expansion coefficient along the a axis direction and a large contribution of a negative thermal coefficient along the c axis direction.
A problem has remained that the glass-ceramics substrate has, however, a large hysteresis in thermal expansion which causes a hysteresis of a center reflective wavelength of an FBG to be large, with the result of a great change in center reflective wavelength of an FBG according to a change in temperature. Note that a hysteresis in thermal expansion shows a phenomenon that non-coincidence arises between behaviors in the courses of a rise and fall in temperature where a material stretches and shrinks according to a change in temperature.
Contrast to this, a method is disclosed in Japanese Patent Laid Open No. 2000-503967, in which a heat cycle treatment is performed at a temperature of 400 to 800xc2x0 C. in order to reduce a hysteresis in thermal expansion of a glass-ceramic substrate to stabilize an internal structure, whereas a hysteresis in thermal expansion reduced in such a method is still unstable to changes in environment such as temperature and humidity and an initial value is difficult to be maintained. Additionally, such a heat treatment causes a fabrication process to be complex, thereby leading to a problem to increase in cost.
In WO 01/04672, a disclosure is given in which if a athermal member that is made of a polycrystalline body (ceramic made of a sintered body of powder) having a major crystal of xcex2-quartz solid solution or xcex2-eucryptite solid solution, a spacing between crystal planes thereof that gives a major diffraction peak in X ray diffraction measurement being smaller than 3.52 xc3x85 and a negative thermal expansion coefficient is used as a substrate of an FBG, not only can a temperature dependency of a center reflective wavelength of the FBG be suppressed, but a hysteresis in thermal expansion is also reduced. Note that since this ceramic has clearances in grain boundaries in the interior thereof and further has xcex2-quartz solid solution or xcex2-eucryptite solid solution showing a behavior of anisotropic thermal expansion, the ceramic has a negative thermal expansion coefficient due to a mechanism similar to the above glass-ceramics.
However, when a device using the above glass-ceramic or ceramic as a substrate exposed to a high temperature and high humidity environmental atmosphere for a long period, they absorbs water into the interior of a substrate, microcracks and clearances in grain boundaries necessary for attaining a negative expansion characteristic are filled with a reaction product between water and a substrate, and as a result, a thermal expansion coefficient shifts toward a positive direction, which has led to a problem that a device from such a substrate is hard to maintain prescribed performance as the device.
Contrast to this, in Japanese Patent Laid Open No. 2000-327372, a disclosure is given in which a surface of a glass-ceramic substrate is coated with a solution containing a silane given by the following formula (5) to avoid water to put into contact with the substrate, thereby enabling solution of the above problem.
R6 Si (OZ)3 . . .xe2x80x83xe2x80x83(5) 
wherein R6 is a hydrocarbon group in which F atom may be contained and having 1 to 10 carbon atoms, and Z is a monovalent hydrocarbon group containing methyl or ethyl group.
In a wavelength division multiplex transmission optical communication system, more of light is required for being multiplexed in order to transmit more of information and while in company with this trend, a request has been made for further reducing temperature dependency of a center reflective wavelength of an FBG; and in coping with this request, even if a silane solution disclosed in Japanese Patent Laid Open No. 2000-327372 is used, a water repellency on a glass-ceramic substrate is still insufficient, a thermal expansion coefficient of the glass-ceramic substrate showing a negative thermal expansion varies, though, slightly if exposed to a high temperature and high humidity environmental atmosphere for a long period, which results in a problem of insufficient temperature dependency of a center reflective temperature of an FBG.
In a case where an optical component with a positive thermal expansion coefficient, for example an FBG, is fixed on an optical communication device substrate using an adhesive made of a polymer adhesive having high productive efficiency, especially, an epoxy resin, silicone resin, acrylic resin or the like, an adhesive force of a adhesive is reduced or no force thereof is exerted if the solution containing a silane of the formula (5) is employed as a treatment agent for the substrate, thereby disabling fixing of an optical component on the substrate in a stable manner.
The present invention has been made in light of the above circumstances and it is accordingly an object of the present invention to provide an optical communication device substrate, capable of performing a water repellency treatment in a short time, made of ceramic or glass-ceramic showing a negative thermal expansion, and with almost no change in thermal expansion coefficient even if exposed to a high temperature and high humidity atmosphere for a long period, thereby reducing a hysteresis in thermal expansion; and an optical communication device capable of strongly fixing an optical component onto the substrate even if a polymer adhesive is used.
The present inventors made clear a problem that since although a silane solution was easy to penetrate into the interior of a substrate, a polymerization rate was slow and the solution vaporized before reaching a sufficient level of polymerization, a film thickness necessary and sufficient for suppressing water penetration was unable to be obtained and water, though of a small amount, penetrated into the interior of the substrate, reducing an effect to suppress a change in thermal expansion coefficient, in light of which in order to attain sufficient repellency with the silane solution, a necessity existed for an increased number of silane treatments, which led to inefficiency; and then has further found that the above object was achieved by use of a solution containing at least one organic silicon compound selected from the group consisting of siloxane compounds and silazane compounds instead of the silane solution, leads to proposal of the present invention.
That is, an optical communication device substrate of the present invention is directed to an optical communication device substrate made of ceramic or glass-ceramic having a negative thermal expansion coefficient in the range of xe2x88x9210 to xe2x88x92120xc3x9710xe2x88x927/xc2x0 C. in a temperature range of xe2x88x9240 to 100xc2x0 C., treated with a solution containing at least one organic silicon compound selected from the group consisting of siloxane compounds and silazane compounds.
An optical communication device of the present invention is directed to an optical communication device obtained by fixing an optical component having a positive thermal expansion coefficient onto a substrate made of ceramic or glass-ceramic having a negative thermal expansion coefficient in the range of xe2x88x9210 to xe2x88x92120xc3x9710xe2x88x927/xc2x0 C. in a temperature range of xe2x88x9240 to 100xc2x0 C., wherein the substrate is treated by a solution containing at least one organic silicon compound selected from the group consisting of siloxane compounds and silazane compounds.